Fix link to NUMFOCUS CoC
Fix link to NUMFOCUS CoC

@cpcloud I sent link in MD format, sorry ... this PR fix this link
issue.
Author: Ivan Ogasawara <ivan.ogasawara@gmail.com>

Closes #1884 from xmnlab/fix-link-coc and squashes the following commits:

701ba97 [Ivan Ogasawara] Fix link to numfocus coc
ibis-project/ibis,cpcloud/ibis,cloudera/ibis,cpcloud/ibis,ibis-project/ibis,ibis-project/ibis,cloudera/ibis,cpcloud/ibis,cloudera/ibis,ibis-project/ibis,cpcloud/ibis